1. Field of the Invention
The present invention relates to a light emission circuit capable of controlling luminance of a light emitting diode (hereinafter called an LED) or the like in a photoelectric smoke detector of the like.
2. Description of the Related Art
FIG. 2 is a diagram showing the arrangement of a conventional light emission circuit.
The light emission circuit emits light with desired luminance from an LED 1 in the photoelectric smoke detector. The light emission circuit has a light emission control unit 2 formed by a microprocessor or the like. The light emission control unit 2 is connected to a digital-to-analog converter (hereinafter called a D/A converter) 3 to which a digital value DV for controlling the luminance of the LED 1 is supplied from the light emission control unit. The D/A converter 3 converts the digital value DV supplied from the light emission control unit 2 into an analog voltage AV and outputs the analog voltage. The output side of the D/A converter 3 is connected to the positive (+) input terminal of an operational amplifier 4. The output side of the operational amplifier 4 is connected to the base of a transistor 5.
A capacitor 6 and a resistor 7 are connected in parallel to the base and the emitter of the transistor 5. The emitter of the transistor 5 is connected to the negative (-) input terminal of the operational amplifier 4. The emitter of the transistor 5 is connected to a ground voltage GND through a resistor 8 and the collector thereof is connected to the cathode of the LED 1. The anode of the LED 1 is supplied with power supply voltage VP.
In such a light emission circuit, when the light emission control unit 2 outputs the digital value DV corresponding to desired luminance, the D/A converter 3 converts the digital value DV into the analog voltage AV corresponding thereto and supplies the analog voltage to the operational amplifier 4. The analog voltage AV is converted into a predetermined driving current DI by a voltage/current conversion circuit formed by the operational amplifier 4, a transistor 5, a capacitor 6 and resistors 7, 8. The driving current DI thus converted flows into the LED 1 through the transistor 5, whereby the LED 1 emits light with the luminance corresponding to the digital value DV.
However, the conventional light emission circuit has the following problems.
When the driving current DI for the LED 1 is controlled discretely at every 1 mA or on a 1 mA unit basis within a range of 0 to 800 mA, for example, in order to obtain accurate luminance in a wide range, the D/A converter 3 is required to have high accuracy and resolution of 800 levels.
For example, if the analog voltage AV outputted from the high accuracy D/A converter 3 is in a range of 0 to 2 V, the voltage change width per one level is 2.5 mV . In this respect, since the operational amplifier 4 has an offset voltage of about 10 mV in general, an error of the driving current DI becomes large in particular at the low level thereof, so that it was difficult to drive the LED 1 accurately.